Can't feel Mirena strings?

Hey I've had my IUD since the beginning of April 2011 and I was able to feel my strings for some time. When I checked yesterday though I could no longer feel them. My husband suggested that it may have moved because I had just had my period. He also said that he felt it during intercourse, but could not feel the strings when I asked him to check as well ( Sorry for TMI) Is my period a factor or should I call my midwife?